You can RECORD events and GROUP BY the date they occurred.
Example:
SELECT eventDate, COUNT() as 'events'
FROM events
GROUP BY eventDate
ORDER BY eventDate ASC
, :
2009-08-11 | 23
2009-08-09 | 128
2009-08-06 | 15
EventDate, substr() , :
SELECT substr(eventDate, 1, 10) as 'date', COUNT() as 'events'
FROM events
GROUP BY substr(eventDate, 1, 10)
ORDER BY eventDate ASC